Question [Q] Native video call data consumption

I am going to get 2 i8190Ns, and I want to know how much bandwidth is used during video calls
taken with the built in option, to pick the right data plan. (I'm not asking you to recommend me a plan )
I know Skype and Tango use around 4.5MBp/s, but this info is not enough since guesstimating the relation to
the native function is not satisfying.

Does anyone have a bit more accurate info?